Method for inspecting a connecting surface of a flip chip

ABSTRACT

The present invention provides a method for inspecting a connecting surface of a flip chip to solve problems that the grinding, polishing and chemical etching method is used for making a sample. The present invention utilizes ion beam etching technology for making and processing a sample of the flip chip (FC). The ion beam etching technology includes two modes: keeping the energy of ion beam and increasing the etching time; and keeping the etching time and increasing the ion beam energy. The ion beam etching technology can remove a deforming portion between the solder ball and the metal pad, which is connected thereto because of the grinding and polishing. Specially, it is easy to analyse a sample of a scanning electron microscope (SEM) which includes an intermetallic compound formed between the solder ball and the metal pad connected thereto.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for inspecting aconnecting surface of a flip chip, and more particularly to a method formaking a sample of a flip chip.

BACKGROUND OF THE INVENTION

[0002] Conventional package processes of the integrated circuit (IC) isused for one set of one and includes the following steps: providing alead frame or a substrate; attaching a die (chip); providing a pluralityof bonding wires; molding the die; and trimming and forming a package.The size of the packaged integral circuit is several times bigger thanthat of the chip. The flip chip is provided with a bump which is made ofCu or solder for soldering the chip to a printed circuit board (PCB).When the flip chip is mounted on the PCB, the active surface of a flipchip faces down (hence the name).

[0003] A connection of the flip chip is also known as a controlledcollapse chip connection (C4 connection) and developed by the IBMCorporation of U.S.A. The flip chip connection belongs to a connectiontechnology of the area array, and does not belong to connectiontechnologies of a wire bond and a tape automated bonding (TAB) that onlyprovides a connection of the peripheral array. Thus, the flip chipconnection can be applied to a process and packaged with much higherwire density. The proportion of the connection technology of the flipchip to the future connection technology of the package with higher wiredensity will be large.

[0004] The principle of flip chip connections is that solder bumps arefirstly formed on the metal pad of a chip, the chip is put on asubstrate, the solder bumps the chip are aligned with pads of thesubstrate, and the chip and substrate are processed by means of heattreatment of reflow. After the solder melted, it forms a ball by meansof surface tension, such that the connection between the chip and thesubstrate is complete.

[0005] There are many types of solder. Typical solders are as follows:an alloy (95% Pb and 5% Sn) with high melting point, an alloy (51%indium, 32.5% bismuth and 16.5% Sn) with low melting point, an alloy(63% Pb and 37% Sn) with low melting point and an alloy (50% Pb and 50%indium) with low melting point. During solder bump manufacture, a chipis coated with a passivation layer for sealing and keeps the solder dry.After a hole is formed and disposed above a metal pad, a multi-layermetal thin film (general called as under bump metallurgy) with Cr, Cuand Au is sputtered for providing adhesion and diffusive obstruction,increasing the wetness of solder and avoiding oxidation. Then, an alloy(Pb and Sn) with 100˜125 μm is formed by means of technology ofevaporation, dipping or ultrasonic soldering. During subsequentprocesses of heat treatment of connection, the solder layer will form aball-shaped solder bump by means of surface tension after the soldermelts. Conversely, a pad of a substrate must also be plated with amulti-layers metal thin layer (called as top surface metallurgy, TSM)for providing the wetting of connection of the solder bump.

[0006] The reliability of soldering a connection between differentmaterials is an important consideration of the reliability of anelectronic product. Among solder material, upper and lower main materialto be soldered, atoms is moved to induce the reaction between interfacesand further an intermetallic compound (IMC) is formed because ofdifferent chemical formulas. The intermetallic compound which isgenerated is the main factor of crack and damage the solderingconnection. All the thickness, kind, shape and composition of theintermetallic compound are formed by using the heat treatment greatlyaffect the electrical performance and reliability of the flip chip.Thus, it is very important for analysis work to make a clear photo usingscanning electron microscopy (SEM) for understanding the reactionbetween interfaces of the intermetallic compound.

[0007] In addition, the stress which is generated by the differencebetween the coefficient of thermal expansion of the chip and thesubstrate, and the fatigue which is generated by the repeated operatingtemperature are also the main factors of damage of the solderingconnection of the solder bump of the flip chip. The fatigue damage ofthe soldering connection includes a mechanical fatigue and a thermalfatigue, and the latter is a major factor.

[0008] The chemical composition, mechanical property, height, shape andgeometric arrangement of the solder are also important factors ofsoldered connection lifetimes except the difference between thecoefficient of thermal expansion and repeating operating temperature.Thus, resisting the fatigue of the soldering connection has been animportant research subject for improving the reliability of theconnection of the flip chip.

[0009] In a conventional material analysis technology, a scanningelectron microscopy should be the most used. The scanning electronmicroscopy includes an electron gun that emits an electron beamdownwards. The electron beam is focused through a set of condenser lens,passes through a set of scanning coil of controlling electron beam byusing a condenser aperture to select the beam size, and then is focusedon a sample through an objective lens. A signal receiver device isdisposed above the sample for selecting secondary electrons orbackscattered electrons to form an image.

[0010] According to the sample which is used in the scanning electronmicroscopy, conventional processing method removes a deforming layer ofthe sample by utilizing a chemical etching method. The chemical etchingis a selective etching method, and therefore different chemicalsolutions which are used to etch the deforming layer depend on differentkinds of materials and the composition to be researched. In other words,a specific chemical solution only etches a specific material orcomposition. Thus, the sample includes various elements or phases thatcan have a problem of over-etching of some element or phase by utilizingthe chemical etching. Careful consideration is required to the dangersof making the chemical solution and the effects of polluting theenvironment.

[0011] Taiwan Patent No. 320,020 entitled “Making Technology Of Sampleof An Electron Microscopy Of A Thin Film Transistor”, discloses that asample of an electron microscope is made by utilizing grinding andpolishing technology. However, after cutting, grinding and polishing, adeforming layer can be formed on a typical sample that affects theinspection of the SEM photo.

[0012] Accordingly, there exists a need for a method of inspecting aconnecting surface of a flip chip to solve the above-mentioned problemsand disadvantages.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide a method forinspecting a connecting surface of a flip chip.

[0014] It is another object of the present invention to solve problemsof a chemical etching method for making a sample of a scanning electronmicroscopy.

[0015] In order to achieve the foregoing objects, the present inventionprovides a method for inspecting a connecting surface of a flip chip formaking and processing a sample according to a connection structure of aflip chip (FC). The method includes the following steps: cutting theconnecting structure of the flip chip, then forming a thin flip chipafter grinding and polishing; and processing a surface of the thin flipchip so as to get the necessary sample of a scanning electron microscope(SEM) by an ion beam etching method.

[0016] The present invention discloses a method that utilizes the ionbeam etching method for processing a surface of the sample of the flipchip. The ion beam etching method mainly controls two conditions: theenergy of the ion beam and etching time for processing the surface ofthe sample. The follow advantages are:

[0017] (1). the ion beam etching method does not belong to the selectiveetching method and suitably applies to a sample with multi-material ormulti-phase;

[0018] (2). the energy of an ion beam, the magnitude of current and thetype of reactive gas can be adjusted and therefore the ion beam etchingmethod is suitably applied to precise etching with a low etching rate soas to decrease damage to the sample;

[0019] (3). the control of process parameters is precise and convenientand the reappearance of condition of the sample is also high; and

[0020] (4). the ion beam etching method can be applied to the etching ofsmall area for etching a specific area of the sample.

[0021] The foregoing, as well as additional objects, features andadvantages of the invention will be more readily apparent from thefollowing detailed description, which proceeds with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a connecting structural view of a single solder ball anda metal pad connected thereto according to a flip chip technology.

[0023] FIGS. 2A˜2E are views of SEM image of a first sample of thepresent invention showing the SEM image that the energy of ion the beamis kept, the etching times are set in different time, and the SEM imageis not etched yet.

[0024] FIGS. 3A˜3E are partially expanded view of SEM image respectivelycorresponding to FIGS. 2A˜2E.

[0025] FIGS. 4A˜4B are views of SEM image of the first sample accordingto a first preferred embodiment of the present invention.

[0026] FIGS. 5A˜5E are views of SEM image of a second sample of thepresent invention showing the SEM image that the energy of ion beam isdifferent, the etching times are set to same time, and the SEM image isnot etched yet.

[0027] FIGS. 6A˜6E are partially expanded views of SEM imagesrespectively corresponding to FIGS. 5A˜5E.

[0028] FIGS. 7A˜7B are views of SEM image of the first sample accordingto a second preferred embodiment of the present invention.

[0029] FIGS. 8A˜8B are views of SEM image of the first sample accordingto conventional chemical etching methods showing the above-mentioned SEMimage of the sample of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Referring to FIG. 1, a single solder bump, the connectingstructure of a metal pad and a solder pad to be connected with the flipchip connection technology is described in an example hereafter. Asshown in FIG. 1, a dielectric passivation layer 3 with a nitride layeris disposed on a surface of a metal pad 2 (e.g. copper pad) of a typicalsilicon chip 1. A contact hole is formed and connected to the metal pad2. A multi-layer metal thin film (known as top surface metallurgy, TSM)is formed in the contact hole for wetting the solder bump connection. Amethod for making the multi-layer metal thin film includes the followingsteps: forming metal layers Ti 4 and Cu 5 in sequence by utilizing aphysical vapor depositing method; forming a Ni layer 6; forming a solderball 7; and completing a so-called solder bump. A solder pad 9corresponding to the above-mentioned solder bump and providing a wetconnection is formed, a surface of substrate (PCB chip) 8. Another Nilayer 91 is located on a surface of the solder pad 9. The flip chipconnection method includes the following steps: aligning the solder bumpof the flipped chip 1 with the solder pad 9 of the substrate 8; and thenforming the connection by means of the reflow method.

[0031] The reliability of soldering connections between differentmaterials is an important consideration of the reliability of anelectronic product. Among solder material, upper and lower main materialto be soldered, atoms are moved to induce a reaction between theinterfaces and further an intermetallic compound (IMC) is formed becauseof the different chemical formulas. The generated intermetallic compoundis the main factor of crack and damage for soldered connections. Thethickness, type, shape and composition of the intermetallic compound areformed by using heat treatment greatly affecting the electricalperformance and reliability of the flip chip. Thus, it is very importantfor analysis work to make a clear photo of scanning electron microscopy(SEM) to understand the reaction between interfaces of the intermetalliccompound.

[0032] The present invention basic includes two following steps:

[0033] (1). cutting a connecting structure of the flip chip shown inFIG. 1, then forming a thin flip chip having a thickness being between2.5 and 3 mm after grinding and polishing;

[0034] (2). processing a surface of the above-mention grinded polishedthin flip chip so as to get a necessary sample of a scanning electronmicroscope (SEM) by an ion beam etching method.

[0035] The present invention utilizes the ion beam etching method thatmainly controls two conditions: energy of ion beam and etching time forprocessing the surface of the sample and then removing a deformingportion between the solder ball and the metal pad that is connectedthereto because of the grinding and polishing. Specially, it is easy toanalyse a sample of SEM which includes an intermetallic compound (IMC)formed between the solder ball and the metal pad connected thereto. Thefollowing embodiment is described.

[0036] According to an embodiment of the first condition, the energy ofthe ion beam is kept to 2.5 keV. Argon (Ar) is a reactive gas and an ionresource, five sets of the etching times are respectively set in 3, 5, 7and 9 minutes, the grinded polished thin flip chips are respectivelyetched by the ion beam, SEM images are respectively inspected throughthe scanning electron microscope, and the SEM images are shown in FIGS.2A˜2E. (FIG. 2A shows that grinded, polished thin flip chips that arenot etched by an ion beam.) FIGS. 3A˜3E are partial expanded views whichare respectively corresponding to FIGS. 2A˜2E.

[0037] According to another embodiment of the second condition, theetching time is set to 3 minutes, argon (Ar) is a reactive gas and aresource of ions, the energy of the ion beam is respectively controlledto 3.5 keV, 4.5 keV, 5.5 keV and 6.5 keV, the grinded polished thin flipchips are respectively etched by the ion beam, SEM images arerespectively inspected through the scanning electron microscope, and theSEM images are shown in FIGS. 5A˜5E (FIG. 5A shows that the grindedpolished thin flip chips are not etched by the ion beam). FIGS. 6A˜6Eare partial expanded views which correspond respectively to FIGS. 5A˜5E.

[0038] After the flip chips have been etched by the ion beam accordingto the above-mentioned embodiments in two different conditions, it iseasy to observe an interface between two phases of lead (Pb) and tin(Sn) in the solder ball and to view an intermetallic compound (IMC)which is formed into a wavy shape and disposed between the solder balland metal pad connected thereto. Thus, it is helpful to analyze work ofcomposition and micro-hardness. Furthermore, the thickness of theintermetallic compound can be also precisely measured; the thicknessthereof is about 1 μm in the embodiment of the present invention. Inaddition, a pillar-shaped microstructure of nickel (Ni) can be alsoclearly observed.

[0039] In conclusion, by getting the above-mentioned SEM images underdifferent conditions, we can understand that the condition for gettingthe SEM image is described as follows:

[0040] (1). the energy of the ion beam kept to 2.5 keV and the etchingtime set to 7 minutes (as shown in FIGS. 4A˜4B); and

[0041] (2). the etching time of the ion beam set in 3 minutes and theenergy kept to 4.5 keV (as shown in FIGS. 7A˜7B).

[0042] As compared with the present invention, the prior art utilizes achemical etching method (nitric acid:acetic acid:glycerin=1:1:4, thetemperature is kept to 80 degrees Celsius, and the etching time is setto 3 minutes) to make an SEM image of a sample it is not easy to observean interface between two phases of lead (Pb) and tin (Sn) in the solderball and to observe an intermetallic compound.

[0043] The above-mentioned embodiment is a preferred embodiment only fordisclosing the present invention, and condition parameters of an ionbeam etching method including the energy of the ion beam, the etchingtime and type of the reactive gas of the ion resource to be used, all ofwhich depend on the material of the solder bump and the connected metalpad, thereto in the connection of the flip chip.

[0044] Although the invention has been explained in relation to itspreferred embodiment, it is not used to limit the invention. It is to beunderstood that many other possible modifications and variations can bemade by those skilled in the art without departing from the spirit andscope of the invention as hereinafter claimed.

What is claimed is:
 1. A method for inspecting a connecting surface of a flip chip and for making and processing a sample according to a connecting structure of a flip chip (FC), the method comprising the following steps of: cutting the connecting structure of the flip chip, then forming a thin flip chip after grinding and polishing; and processing a surface of the thin flip chip so as to get the necessary sample of a scanning electron microscope (SEM) by an ion beam etching method.
 2. The method according to claim 1, wherein the thin flip chip has thickness between 2.5 and 3 mm.
 3. The method according to claim 1, wherein the ion beam etching method includes the energy of an ion beam kept at 2.5 keV and an etching time set to 7 minutes.
 4. The method according to claim 1, wherein the ion beam etching method includes energy of an ion beam kept to 4.5 keV and etching time set to 3 minutes.
 5. The method according to claim 1, wherein the ion beam etching method includes energy of an ion beam being between 4 keV and 6 keV and etching time set in 3 minutes. 